1. Field of the Invention
The present invention relates to a method for fabricating an integrated device comprising a structure with a programmable resistance including a solid electrolyte.
2. Description of the Related Art
Demands to data memories are steadily increasing with regard to the density of information, i.e. how much information can be stored per unit area, access time, i.e. how fast a memory element may be accessed, and non-volatility, i.e. whether the memory content can be reliably maintained even without supplying energy. In conventional electronic data memories, such as DRAM or Flash RAM, often a capacitor stores a unit of information, whereas only the latter type, the Flash RAM, is able to hold a memory content over a considerable amount of time without the need of supplying energy.
In this kind of data memories, a distinction is made between a charged capacitor state and an uncharged capacitor state, with these two states representing the possible bits of information “1” and “0”. Apart from capacitors, additional components, such as selection transistors, are necessary for operating the memory. These components are defined by imaging lithographic processes and structuring techniques, such as etching or damascene processes. A very common manufacturing process in the semiconductor industry for manufacturing memory devices, integrated circuits, and microprocessors is the so-called CMOS process, in which all functional components are realized on a single substrate. The complete integration of functional electronic units, including transistors, resistors and capacitors, is obtained by the use of additional material such as metals, dopants and dielectrics.
In order to combine non-volatility with speed and integration, alternatives to the DRAM and the Flash RAM are subject to intense scientific and industrial research. Alternative approaches range from mechanical memories employing scanning probes over optical concepts to ultra-fast magnetic data storage. As far as the integration into existing manufacturing processes and technologies is concerned, the so-called resistive memories are most promising.
In a resistive memory, a local and stable change of the electric conductivity is achieved by electric signals and is read out with an electric current, where, for example, a high and a low resistive state correspond to the information units “0” and “1”, respectively. Prominent members of resistive memory media are the so-called solid electrolytes, in which metal ions are mobile and can hence migrate therein. Charged metal ions can therefore be positioned by means of an electric field to form a conductive bridge in the carrier electrolyte. In this way, they form a domain with an increased electric conductivity in the otherwise insulating carrier electrolyte. These conductive bridgings are stable over long time spans in the range of several years. Furthermore, it is possible to decompose a conductive bridging by applying an electric field with reversed polarity, and the ions are led back into one of the facing electrodes. The entire process is completely reversible, and, moreover, satisfying extrapolated retention times of more than ten years as well as a sufficient endurance of about 105 cycles have already been shown, e.g. by R. Symanczyk, in Proceedings of the Non-volatile Memory Technology Symposium, 17-1, San Diego (U.S.A.), November 2003.
Besides these encouraging results, problems remain, above all, problems of efficient manufacturing of such devices. In recent decades, semiconductor industry has established highly efficient and very reliable manufacturing processes, such as the CMOS process. It is most desirable to be able to integrate new resistive memory media into this process. In this manner, endurance and non-volatility of resistive memory media can be combined with the high integration power of the CMOS process to form a high performance data memory device that holds its information content over a long time span without requiring energy.
Solid electrolytes, however, require additional doping to provide a sufficient mobility of the metal ions at room temperature. As an example, silver can be dissolved in germanium-sulfide or germanium-selenide solid electrolytes to form a good room temperature electrolyte, as discussed at http://www.axontc.com/images/axontc.pdf. Although the addition of silver is very advantageous, as far as the solid electrolyte properties are concerned, a doped solid electrolyte is often rendered inoperative by specific steps of the CMOS production process. Particularly, process steps with elevated temperatures, which are an integral part of the CMOS process for annealing semiconductors or forming passivating seals, cause the dopant in the solid electrolyte to form a stable compound with the constituents of the solid electrolyte.
In the case of silver (Ag) and germanium-selenide (GeSe), for example, a GeSeAg-argyrodite is formed when temperatures above a certain critical value are applied. The said argyrodite does not possess any solid electrolyte properties and holds the metal immobile, in this case silver, at fixed lattice sites. Therefore, it is impossible for the metal ions to migrate and hence to form a conductive bridging. The reason why the mobility is lost lies in the fact that during long exposures to high temperatures of the material system, the dopant material can migrate within the thitherto electrolyte, and can then form stable chemical bonds with the electrolyte to yield a new crystalline phase.
In summary, it is impossible to integrate a doped solid electrolyte into a device, employing a CMOS manufacturing process with an extended heating process for completing the device.